A laser in its simplest form can be schematically illustrated as including a gain medium that is located between two mirrors. Light within the laser cavity is reflected back and forth between the mirrors, each time passing through the gain medium, which produces optical gain. The mirror coating on the first mirror may be totally reflective, while the mirror coating on the second mirror may be partially reflective, thereby permitting some light to escape from the laser cavity. The spatial region between the reflective surfaces of the mirrors defines the laser resonator or cavity, and in the context of the present invention relates to the so-called "intracavity region".
The intensity of the laser output is a function of both the wavelength region over which the gain medium operates and the reflectivity of the resonator elements. Normally this output is broad and without sharp, distinctive spectral features.
The identification of gaseous species, e.g., atoms, molecules, radicals, or ions, via laser spectroscopy requires that the laser output be in a wavelength region where the species absorbs. In conventional applications of lasers to the detection of gaseous species, laser radiation is used to excite a gas sample that is external to the laser in order to produce a secondary signal such as ionization or fluorescence. Alternatively, in conventional absorption spectroscopy, laser light is passed through a gas sample that is situated outside of the laser and attenuation that varies with wavelength is monitored.
Some twenty years ago, another detection methodology, intracavity laser spectroscopy (ILS) was first explored; see, e.g., G. Atkinson et al, "Detection of Free Radicals by an Intracavity Dye Laser Technique", Journal Of Chemical Physics, Vol. 59, pp. 350-354, (Jul. 1, 1973). In ILS, a laser itself is used as the detector. The gas sample to be analyzed is inserted into the optical cavity of a multimode, homogeneously broadened laser. Atkinson et al, supra, showed that by placing gaseous molecules, atoms, radicals, and/or ions in either their ground or excited states inside the optical cavity, the laser output can be altered. In particular, the absorption spectrum of the intracavity species appears in the spectral output of the laser.
Distinct absorption features in the laser output arise from the intracavity losses introduced by the gaseous species that are absorbing. (As used herein, an absorption feature corresponds to a series of consecutive wavelengths where the light intensity reaches a single local minimum in light intensity in a plot of light intensity versus wavelength.) In a multimode laser, intracavity absorption losses compete with the laser gain via the normal mode dynamics. As a result, attenuation can be observed in the laser output intensity at wavelengths where the stronger intracavity absorption features compete effectively against the gain of the laser. The more intense the absorption features, the larger the decrease in the laser output intensity at those wavelengths.
By inserting the absorbing gaseous species inside the laser resonator, ILS can provide a detection sensitivity that is enhanced over conventional spectroscopy methods. The enhanced detection sensitivity of ILS techniques derives from the non-linear competition between (1) the gain produced in the laser gain medium and (2) the absorber loss(es). As a result, ILS can be utilized to detect both weak absorption and/or extremely small absorber concentrations.
Each gaseous species in the optical cavity can be uniquely identified by its respective absorption spectrum or signature. Additionally, the intensity of a specific absorption feature or features in the spectral signature can be used to determine the concentration of the gaseous species once the sensor is appropriately calibrated. (As used herein, the term "spectral signature" corresponds to the wavelength plotted against absorption intensity or absorbance that uniquely identifies the gaseous species.)
The spectral signature of the gaseous species can be obtained by dispersing the output of the ILS laser with respect to wavelength. Two detection schemes are typically employed to disperse the output of the ILS laser and thereby obtain the spectral signature of the gaseous species. The output of the ILS laser can be passed through a fixed-wavelength, dispersive spectrometer, and the specific spectral region that is resolved by this spectrometer can be recorded using a multichannel detector; see U.S. Pat. No. 5,747,807, issued May 5, 1998, to G. H. Atkinson et al entitled "Diode Laser-Pumped Laser System for Ultra-sensitive Gas Detection via Intracavity Laser Spectroscopy (ILS)". Alternatively, a spectrometer that can be scanned in wavelength can be employed to selectively resolve different spectral regions that are recorded with a single channel detector, supra.
Prior art ILS detection systems employ ILS lasers having a spectral bandwidth that is substantially broad relative to the bandwidth of the absorption features in the absorption spectrum of the intracavity species to be detected; see U.S. Pat. No. 5,689,334, issued Nov. 18, 1997, to G. H. Atkinson et al entitled "Intracavity Laser Spectroscope for High Sensitivity Detection of Contaminants". In particular, the laser systems possess an operational wavelength bandwidth that is at least three times as broad as the absorption features of the gaseous species being monitored.
Prior art methods of performing ILS, however, while successfully demonstrated in the laboratory, are too large and complex for many commercial applications. In particular, the requirement for a spectrometer to disperse the spectral output of the laser, as well as for a computer to analyze the absorption features, adds to the size and complexity of the detection system. In contrast, the constraints of commercial reality dictate that a gas detector be conveniently sized, relatively inexpensive, and reliable.
One approach used to measure contaminant concentration based on temporal characteristics is called "cavity ringdown". Such a technique allows optical absorption measurements to be made using a pulsed light source and offers a sensitivity that is greater than that attained using stabilized continuous light sources. The technique is based upon the measurement of the rate of absorption rather than the magnitude of absorption of a light pulse confined within a closed optical cavity. Examples of cavity ringdown are described, for example, in A. O'Keefe et al, "Cavity ring-down optical spectrometer for absorption measurements using pulsed laser sources", Reviews of Scientific Instruments, Vol. 59, No. 12, pp. 2544-2551 (December 1988); J. J. Scherer et al, "Cavity Ringdown Laser Absorption Spectroscopy: History, Development, and Application to Pulsed Molecular Beams", Chemical Reviews, Vol. 97, No. 1, pp. 25-52 (Feb. 5, 1997); U.S. Pat. No. 5,528,040, issued Jun. 18, 1996, to K. K. Lehman; and U.S. Pat. No. 5,815,277, issued Sep. 29, 1998, to R. N. Zare et al.
Cavity ringdown laser absorption spectroscopy is a well-known and successfully utilized technique for detecting trace contaminants in the gas phase. It differs, however, in several respects from the method of the present invention. (1) Cavity ringdown is implemented in a passive optical resonator, i.e. there is not an active optical element such as a gain medium. In contrast, the present invention relates to measurement and characterization of temporal phenomena inside an active optical resonator, specifically, a laser resonator. (2) The photon pulses exiting a cavity ringdown resonator are spaced periodically by the round trip time of the laser pulse injected into the passive resonator. In contrast, the temporal phenomena of the present invention may or may not include photon pulses at all. In the event that photon pulses are produced and measured, they would not be spaced by the cavity round trip time except as a coincidence. (3) The photon pulses exiting a cavity ringdown resonator decay towards zero output intensity. In contrast, in the event that photon pulses are produced in the present invention, they decay to a steady-state non-zero output intensity given by the continuous wave (cw) output power of the laser. (4) In cavity ringdown, the presence of an absorber does not change the period of the photon pulses exiting the passive resonator, as this is determined by the round trip time determined by the speed of light and the cavity length. In contrast, the presence of an absorber in the present invention may, and usually does, change the period of photon pulses exiting the active resonator.
Thus, there remains a need for a methodology that significantly reduces (1) the complexity of ILS measurements and (2) the size of ILS instrumentation, for example, by eliminating the need for a spectrometer and a computer.